It sounds like you’re talking about Qt Designer, since it defaults to showing two tabs (called “Tab 1” and “Tab 2”) when you add a QTabWidget through the interface.
If so, click on the tab you want to rename, then in the Property Editor (if you can’t find it make sure it’s visible by using the View->Property Editor menu item) scroll down to the bottom and look for the currentTabText property. You can change the tab’s name right there.